Site Report – Mainline Demolition and the Broun Avenue bridge replacement
01 February 2023
In 2021, Tonkin Gap Alliance was awarded the contract to design and construct the Broun Avenue Bridge Replacement Project in Perth, Western Australia.
Scope of the works included partial construction of the new bridge and bus station to divert traffic from existing bridge, before demolition of the old structure.
Mainline chose a mechanical in situ demolition approach over lift and shift option after a comprehensive risk analysis and detailed discussions with the client. The principle heavy lift risks included design and installation of temporary support works, time allocation for cutting eight 1.2 x 1.4 m (4 ft x 4 ft 7 in) PT (post-tensioned) beams during deconstruction, downsizing large bridge sections after removal, eccentric loading of support columns during partial beam removal and more.
These risks including lift study risks such as, geotechnical, and catastrophic collapse were eliminated by using a break in situ system which could continue during night works within noise curfew limits.
Innovative demolition approach
The key innovative approach was a combination of pre-cutting deck slab, after removal of asphalt, and selective overnight deck removal, with concrete crackers working at ground level - removed from the elevated bridge deck - within noise limitations, combined with a flood of plant and equipment and additional back up machinery working the full demolition face.
Mainline deployed a 95 t unit, to process large beams and abutment sections, supplemented by four 30 t class excavators and crackers/shears maintaining an unbroken 24-hour cycle. Several rock-breakers assisted in the day programme where needed.
Demolition scope of the project
The demolition scope encompassed
- Entire removal of the bridge structure
- Traffic management plan for Tonkin Highway closure
- Noise management plan
- Install and maintain barriers, roadway protection demolition of bridge superstructure, abutments, capping beams, all footings and retaining walls
- Remove sufficient sand for access
- Removal of all debris and scrap as well as site clean-up and demobilisation
- Maintain without compromise all flora, fauna and built form abutting bridge and roadway
- All the above completed within the closure period 10 pm on 1 April 2022 to 6 am on 4 April.
Bridge demolition methodology
The Mainline demolition in situ method focused on elimination of risk by removal of people from the danger zones and simply delivered by numbers of machines to execute a well-rehearsed plan and get the job done safely.
This suited the 24-hour shift job profile. Mainline specifically determined not to include high risk rigging and lifting because of the high percentage of works undertaken on night shift.
While Mainline considered the client proposition to close northbound and southbound lanes on separate weekends for heavy lift demolition, ultimately this method was too high risk.
It required considerable engineering and installation time for temporary works, after first confirming (if possible) that the cables were fully grouted. It also required expensive and time-consuming wire saw cutting of post tensioned beams deck cutting, all in a timely manner to meet lift schedules. Furthermore, it was a method which could be easily shut down by inclement weather and storms. As fate would have it rained much of the weekend with some very high wind conditions. The weather conditions alone, would have made it impossible for the heavy lifts to have taken place.
Ultimately simple, low risk methods were adopted and worked both safely and extremely effectively.
Demolition in situ methodology and processes created by Mainline allowed continuous 24 hour works within the residential precinct and eliminated the following:
- Segmentation, rigging and heavy lifting of post tensioned beam sections
- Heavy lifting of estimated eccentric loads and reliance on engineered solutions.
- Working at height for concrete cutters, riggers and others preparing lift sections
- Time-consuming high risk cutting 1.2 x 1.5 m (4 to 5 ft) post tensioned concrete beams
- The possibility of catastrophic lift and temporary works failure
- Follow-on transport, handling, and disposal of large sections and/or breaking down large sections after lifting.
All deck cutting was done on fully enclosed and unbroken deck surface to ensure critical safety.
Meeting the challenges of the project
Successful implementation of the project required Mainline to overcome several challenges.
Covid lockdown and government mandate impacted personnel availability and eligibility restricted the number of personnel available for the project. It also presented a challenge in acquiring sufficient resources to complete the project in a limited supply market,
Additional personnel were fully site inducted with required accredited skills and placed on standby to accommodate shortfalls. Demolition planning and sourcing of resources and equipment was comprehensive to prevent project delays and budget alterations. The equipment was also multi-proposed to ensure maximum use was made of the time and availability scheduled.
To meet the client’s schedule and expectations to remove the bridge spanning the highway in a limited timeframe, Mainline scheduled surplus resources in advance and the project was in fact finished four hours earlier than planned.
Mainline made no claims for variation of contract value and successfully saved the client with their advanced methodology as opposed to the Rig and lift method which would have increased the cost an additional A$300,000 (US212,000).
Opening ahead of schedule
To ensure Tonkin Highway was opened ahead of schedule, the removal of debris and structures adjacent to the highway was reserved for the following week. However, Mainline was able to achieve 90% of this work within the shutdown window, for improved safety of operations to adjacent high-volume traffic.
Additionally Mainline undertook unscheduled excavation of 5,000 cubic metres (177,000 sq ft) of embankment sand to facilitate removal of abutments, all during the demolition shutdown window, pushing the overall earthworks programme seven days ahead of schedule.
Low noise methodology
With noise restrictions limiting rock breakers use to the hours of 7 am to 9 am, Mainline planned low noise methodology to maintain 24-hour productivity without rock breakers. The back-up equipment provided additional support and continual operations despite machine breakdowns and complications that arose during the shutdown.
The demolition management plan allowed for the parallel operations to remove sand embankments, abutments, central column footings, all steel, and debris from the median and road verges, within the closure window.
All the demolition works were mechanical, eliminating temporary support design and installation. And removing rigging and labour personnel from the workface.
The process had begun with extensive planning concerning the road closure and bridge demolition. Planning included 24-hour mechanical backup, despite all machine’s being serviced, and in running order at the outset. This precaution became a critical part of the plan when minor mechanical breakdowns and hydraulic hose ruptures occurred.
Immediate repair and back up machines maintained full production, keeping the programme ahead of schedule, while Mainline processes achieved completion safely, with no lost time injuries, within budget and ahead of the agreed timeline.